Gas-purged ionizers and methods of achieving static neutralization thereof

ABSTRACT

A small quantity of electron attaching gas is introduced into the corona region of an electrical ionizer to stabilize the corona in the otherwise electron non-attaching nitrogen gas. The corona region is closely localized at emitter points so the quantity of electron attaching gas is very small. Clean-dry-air is preferably used as the purge gas but other gases such as oxygen and carbon dioxide may be used. The small quantity of electron attaching gas may be introduced either through a hollow needle emitter or an external purge gas (sleeve about the needle, or by using a gas purge nozzle).

RELATED APPLICATIONS

The present application claims priority of U.S. Provisional ApplicationSer. No. 60/113,684, filed Dec. 22, 1999, entitled “Static Neutralizerfor Thermal Cycling Chamber” and Provisional Application Ser. No.60/113,685, filed Dec. 22, 1999, entitled “Gas-Purged Ionizer”, thedisclosures of which are incorporated by reference herein in theirentirety.

TECHNICAL FIELD

The present invention relates generally to electrical ionizers thatproduce stable charge-carrier production in gases with varyingconcentrations of electron attaching components. More particularly, theinvention relates to ionizers suited for production test environments ofsemiconductor devices and component handlers and other environments thatmight be rendered inert by nitrogen and noble gases.

BACKGROUND ART

In semiconductor component testing, the temperature is normally closelycontrolled at selected values in the range from −60° C. and +160° C.Cooling is accomplished by the introduction of liquid nitrogen and itscold vapors into a test chamber at ambient pressure. Nitrogen gas whichevolves from evaporative cooling is not electron attaching and, as aresult, has a profound effect on electrical ionizers both in stabilityand generation of EMI/RFI.

A primary object of this invention is to produce a balanced amount ofnegative and positive charge carriers for charge neutralization byinjecting a small amount of an electron attaching gas into an electricalionizer to restore and stabilize negative ion production.

Charge imbalances in semiconductor testing equipment are known to resultin electrical discharges that will damage the devices and componentsbeing tested.

Accordingly, another object of this invention is to eliminate suchcharge imbalances in environments where conventional electrical ionizersfail or are difficult to control.

Conventional electrical, x-ray, ultraviolet, and nuclear (radioactive)static eliminators have been used in this application. It has been foundthat conventional electrical static eliminators were unreliable andthose based on ionizing radiation are difficult to control andunacceptable in some markets by their hazardous nature or burdened bylicensing requirements.

Ionizers for static eliminators provide positive and negative chargeshaving the mobility needed to be drawn to static (stationary or fixed)electrical charges on surfaces or charged floating conductors. Theproduction of charge carriers is critical to static elimination.Ionization can be achieved by means of ionizing radiation (primarilyradioactive, x-ray, and ultraviolet sources) and electrical corona.

The primary processes in ion production are ionization itself andelectron attachment. In the ionization process, electrons are separatedfrom a neutral atom or molecule. This action produces positive ions andfree electrons.

In a positive corona the ionization process takes place near anelectrode region with a positive polarity (a deficiency of electrons).The free electrons that are produced in the ionization process are drawnto this corona electrode (either as free electrons or attached asnegative ions). The positive ions have relatively low mobility whencompared to the electrons. The positive ions become available for staticelimination by providing a gaseous ion current of charge carriers. Theyalso stabilize the ionization process by providing a buffering electricfield in the corona region. This stability is aside from the manyunderlying corona fluctuations and phenomena known for such corona.

Ionization proceeds by similar methods with negative corona. However,the free electrons drift away from the corona electrode at highspeed—free electrons typically have a mobility 100-1000 times those ofions. The positive ions that are produced in negative polarity coronaare drawn to the nearby negatively charged corona electrode. In orderfor the corona to be stabilized and for negative ions to be madeavailable for neutralization, the free electrons must attach to neutralatoms or molecules to form negative ions. It is considered known in theprior art that unless only negative charge is to be neutralized, thesuccessful operation of an electrical static eliminator requires anionizable gas and one that is electron attaching.

High purity nitrogen and the noble gases are not electron attaching. Thepresent invention offers methods to achieve balance in gases withcompositions that are dominated by electron non-attaching components,and in chambers with uncontrolled variability of mixtures of electronattaching and non-attaching gases. The present invention is not limitedto the case where nitrogen is evaporated to cool component handlers, butis best used in environments where electrical corona is affected byelectron non-attaching gases, i.e., gases with large differences inpositive and negative carrier mobilities.

The present invention provides low-cost static neutralization in gaseswhere the mobility of corona generated positive and negative carrierspecies differ greatly or change over time. The stability is achieved bythe injection of a small quantity of electron attaching gas, such asair, oxygen, or carbon dioxide, in close proximity to the coronaelectrode.

The use of air for purging an ionizer has been contemplated for otherpurposes. R. Mueller, et al. (U.S. Pat. No. 3,111,605) describes adirectly coupled static bar with needles centered in orifices. Thecasing of the static bar is pressurized with air that escapes throughthe annular spaces around the needles. The bar is intended for use inhazardous areas, where the air is used to keep ignitable vapors awayfrom the ionizing electrodes. Similar inventions were patented by others(see, e.g., Can. Pat. No. 856,917 and W. Ger. Pat. No. 885,450). TheCanadian patent was preceded by prior art attempts including extendedrange (blower-like) applications and blow-off of particles. Others havealso considered such externally purged designs for hazardous area use.

One prior effort involves the introduction of a static bar with hollowemitters for extended range use. Another effort used nitrogen-purgedionizing nozzles in a rinser/dryer to control static charge onsemiconductor wafers (U.S. Pat. No. 4,132,567). This effort was notsuccessful as a result of instabilities developed in the ionizer whenused in nitrogen environments.

U.S. Pat. No. 5,116,583 discloses an air-purged emitter for controllingparticle generation in clean rooms. Moisture in air is known to formparticulate contaminants when exposed to corona discharge. In the '583patent, nitrogen, argon, and helium are identified as purge gases. Theprimary use of the invention in the '583 patent is with dc ionizers. The'583 patent does not recognize the role of electron non-attaching gasesin ionizer design and a method of gas injection to achieve ion balance.

U.S. Pat. No. 5,550,703 discloses a particle-free ionization bar withhigh and low pressure plenums to distribute gases to the emitters. Thevelocity of gases was then matched to maintain uniform flow with that ofsuperficial flow within the clean room. A need for balanced ionizationwas identified, but provisions were not incorporated into the devicedisclosed in the '703 patent to achieve this goal; in other words, nomention is made for the special requirements to achieve balance inelectron non-attaching purge gases. Finally, U.S. Pat. No. 5,847,917describes the use of high velocity gases around emitters to render themcontaminant free.

Ionizers based upon electrical corona are basically of three types:direct-coupled alternating current (ac), capacitively-coupledalternating current (ac), and direct current (dc). The dc ionizers canbe operated with continuous or pulsed high voltage on the coronaelectrodes. Ionizers of the ac variety are desirable because the sameemitters are used for both positive-and negative-polarity iongeneration; thereby, a size reduction is achieved. Also, both polaritycarriers are produced at the same distance from the object to beneutralized, and at the same point in space yielding better mixing ofions with the gas stream. Direct current ionizers offer greater controlin ion generation and typically have separate positive and negativecorona emitters. The present invention is operable for both ac and dcionizers.

The instability of alternating-current (ac) corona in nitrogenenvironments provides the most direct evidence of the problems relatedto gases with largely-different positive and negative carriermobilities. In ac corona each emitter electrode is periodically drivenwith positive and negative polarity voltages. Positive ions are producedon the positive polarity part of the voltage cycle and free electronsare produced on the negative voltage cycle. The free electron current isvery high and limits the peak ac voltage before sparkover. When theemitter is directly connected to the high voltage source the peakvoltage is so limited that positive ion generation is unsatisfactoryand, at best, a negative bias is given to objects intended to receivestatic elimination. The use of capacitively-or resistively-coupledemitters in ac ionizers, as disclosed in one embodiment of the presentinvention, limits the free electron current and offers some stability tothe ionizers. The injection of electron attaching gases will stabilizeresistor-, capacitor-, and directly-coupled corona ionizers.

When current limiting is present in an ac ionizer, it is observed that anegative bias remains at the objects to be neutralized. This imbalanceis particularly noticeable when N₂ is used to convey the positive (ion)and negative (free electron) carriers to the charged objects. Theresulting bias voltage is attributed to the large difference in thecarrier mobilities or diffusivities. This effect is independent of theionizer instabilities and imbalances mentioned above and has beenobserved by others (H. Inaba, et al., IEEE Trans. Semicond. Mfg., 5(4),359-67, (1992)).

Overcoming this negative bias voltage is another object of theinvention.

The negative bias and instability in electron non-attaching gases isalso seen with dc ionizers. It can be eliminated by the method of thisinvention.

DISCLOSURE OF THE INVENTION

The primary goal of this invention is to stabilize an electrical ionizeragainst fluctuations in the electron attaching component of the gasaround the emitter. This method of operation is intended for use in testchambers for finished semiconductor devices and components, where theintroduction of air in small quantities is permitted.

In accordance with the present invention, the introduction of a smallquantity of electron attaching gas into the corona region of anelectrical ionizer is found to stabilize the corona in the otherwiseelectron non-attaching nitrogen gas. This corona region is closelylocalized at emitter points, so the quantity of electron attaching gasis very small. In the present invention, clean-dry-air is mostpreferably used for this purge gas. Gases, such as oxygen and carbondioxide, can be used in other applications. The small quantity ofelectron attaching gas may be introduced either through a hollow-needleemitter (syringe) or an external purge gas (sleeve about the needle, orby using a gas-purged nozzle). Simply introducing an uncontrolled flowof chamber gases (containing residual air) over the needles has not beenshown adequate for the application, since it would require a largeamount of dry air at temperatures as low as −60 C.

Still other objects and advantages of the present invention will becomereadily apparent to those skilled in this art from the followingdetailed description, wherein only the preferred embodiments of theinvention are shown and described, simply by way of illustration of thebest mode contemplated of carrying out the invention. As will berealized, the invention is capable of other and different embodiments,and its several details are capable of modifications in various obviousrespects, all without departing from the invention. Accordingly, thedrawing and description are to be regarded as illustrative in nature,and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be understood through consideration of the followingdrawings.

FIG. 1a is a partly sectional, partly schematic representation of ageneric gas-purged, hollow-electrode arrangement constructed inaccordance with the principles of the present invention;

FIG. 1b is a generic gas-purged, shielded-electrode arrangementconstructed in accordance with the principles of the present invention;

FIGS. 2a-1, 2 a-2 show a gas-purged ac static bar in a nitrogenenvironment in accordance with the invention;

FIG. 2b is an illustration of a test arrangement for the ac static barof FIG. 2a;

FIGS. 3-6 are graphs of results from tests conducted with the ac staticbar;

FIG. 7 is an illustration of a gas-purged hollow electrodeionizer-emitter pair; and

FIG. 8. shows the performance achieved with the gas-purged hollowelectrode ionizer-emitter pair of FIG. 7.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention circumvents the deficiencies of conventionalelectrical ionizers as described above by introducing a small quantityof electron attaching gas into the corona region of an ac or dc ionizer.FIGS. 1a and 1 b illustrate the generic arrangements for gas injectionthrough a hollow emitter electrode and about an emitter in a cavity,respectively. The elements of these ionizers are similar.

FIG. 1a shows a cross-sectional view of an electrode assembly 1 for gasinjection through the corona emitter. The assembly may be tubular orlinear. A potential difference (ac, dc or pulsed voltage) is appliedbetween a conductive or semiconductive corona electrode 2 and aconductive or semiconductive counterelectrode 3. The space betweenelectrodes 2 and 3 is filled with insulating material 4, which mayinclude gases and solid materials. The ionizer is placed in a gaseousenvironment 5. The potential difference between the electrodes 2, 3results in large electrical stresses near sharp edges, such as 6.Electrical corona, the localized electrical breakdown of gases, isclosely localized at emitter points, such as 6, and is the source ofgaseous ions from ionizers. When the region 6 is exposed to anenvironment 5 that does not have electron attaching components, freeelectrons are produced and no negative ions are formed. The absence ofnegative ions results in unstable operation of the ionizer. By injectionof a small quantity of electron attaching gas 7, such as air, oxygen, orcarbon dioxide, to the emitter region 6, negative ions form andstabilize the corona. In the hollow-emitter electrode, the injected gasexits at the emitter 8.

The elements of the needle-cavity assembly 9 in FIG. 1b are nearly thesame as for the hollow-emitter assembly 1 in FIG. 1a. In this case thegas injection channel 10 surrounds the corona electrode 2 and theexiting gases 8 envelop the emitter region 6.

In the primary application to semiconductor-component testers andhandlers, clean-dry-air is most appropriately used for this purge gas.

Although the generic electrode arrangements of FIGS. 1a and 1 b showsingle ionizing assemblies, pairs of positive/negative emitterassemblies 1 in accordance with the invention can be used in dcionizers, and both electrodes 2, 3 can be corona emitters. Also, arraysof emitter assemblies 1 are commonly used. Typical arrays areillustrated in the remaining disclosures, but should not be construed aslimiting the design of the ionizer.

PURGING OF AC STATIC ELIMINATORS

A commercially available ac static eliminator 13 can be purged with asmall quantity of clean-dry-air to stabilize and eliminate the imbalancevoltage in accordance with the present invention. The electrodeconstruction is illustrated in FIG. 2a and the arrangement for tests isschematically depicted in FIG. 2b. The corona electrode set, operatingat 60 Hz ac, consists of 18 needle-type emitters 6 within a groundedelectrode casing 2. The needle electrodes 6 are capacitively-coupledthrough a metal ring 11 to the high voltage wire 12 in an insulationsystem within the grounded electrode casing 2. The ionizer is operatedwith ac voltages from 0 to 16 kV peak-to-peak applied to the wire 12.

The ionizer 13 is enclosed in an environmental chamber 15 maintained atatmospheric pressure as depicted in FIG. 2b. The volumetric flow rate 14and temperature for nitrogen in the chamber ranged from 6000 to 10000ml/min and from −10° to 60° C., respectively. Clean-dry-air (0 to 200ml/min) was injected into the ionizer at 16 to determine its influenceon the charge decay and steady-state balance condition. The nitrogen wasintroduced to the aluminum casing of the ionizer 13 through a PTFE tube17 and generally flooded the gap between the emitters 6 and casing 3(see FIG. 2a). Measurements of charge decay time and charge imbalancewere secured using a half-sphere, conductive probe 18 located about 6 cmdownstream from the ionizer 13.

Charge decay time is the time required for a 1000 V potential on theprobe 18 to be reduced to 100 V. The charge on the probe is proportionalto the potential and will have negative or positive polarity dependingon the potential. For example, a negative charge decay time is the timerequired for positive carriers in the gas stream to neutralize a probeinitially charged to −1000 V. If the potential on the probe is allowedto float after grounding, it will reach a steady potential or residualcharge level. This steady state level is called the charge imbalance,residual potential, or unbalance condition.

As the remaining traces of air are removed from the corona emitterregion 6 to create a nitrogen environment, the time required forneutralization of a positive initial charge decreases while it remainsrelatively constant for a negative initial charge (see FIG. 3 andincreasing time a-e). Shorter positive charge decay times in thisinstance result from the replacement of negative ions (formed fromelectron attachment) with higher mobility free electrons.

Increasing gas flow 14 by the ionizer 13 moves more carriers to theprobe region 18 and reduces the charge decay times for positive andnegative polarity charge accumulations. The effect is significantlygreater for the negative (free electron) carriers (see FIG. 4).

Increases in gas flow rate 14 increase the negative residual potentialon the probe 18 to be neutralized (see FIG. 5).

The injection of small quantities of clean-dry-air into the bar 16 inthe manner described above will reduce the residual voltage on the probe18. The dependence of saturation (residual) voltage on seeded air withinionizer bar 16 is shown in FIG. 6. Under steady-state conditions, thepotential on the probe 18 is positive in clean-dry-air and negative in anitrogen environment. Although their remains a small positive bias inthe balance voltage in the air stabilized corona, this imbalance issufficiently stable to be nulled by conventional balance techniques.

The quantity of gas injected into the ac ionizer 13, as described above,can be significantly reduced by controlled injection of air about theemitters. The flooding of the bar casing with air is only illustrativeof the method.

PURGING OF A DC STATIC ELIMINATOR

FIG. 7 is an illustration of an ionizer constructed of parallel needles,one of negative polarity 19 and one of positive polarity 20. Theseneedles 19, 20 are hollow and contain gas flow channels similar to thosedescribed in FIG. 1a and carry a gas from gas plenums 21 and 22,respectively. The electrodes 19, 20 are spaced apart and separated byenvironmental gases 5 that function as the insulation system 4. The darkcircle in FIG. 7 is a schematically depicted structural component of theenvironmental chamber 23 (see FIG. 2b). The ionizer in FIG. 2b hasemitters at 6 where the injected gases exit at 8.

Charge decay data is shown in FIG. 8 in a nitrogen environment 5 withair injected through the hollow emitters 6, 8. The results show similarcharge decay times for positive and negative probe 18 potentials and asmall positive residual potential, as obtained for the ac ionizer. Aswith the ac ionizer, the purpose for the purge gas 7, 8 is to addstabilizing, electron attaching components to, at least, the negativeemitters in the gas stream.

The dc ionizers are especially suited for use in device and componenthandlers that are cooled with liquid nitrogen. The use of smallquantities of electron attaching gases in the negative emitters and/orpositive emitters is permissible in the testing environment. Small gasquantities are desirable so that the introduced gases are at thermalequilibrium with devices under test. Further, the gases introduced intothe emitter region must be clean and dry to prevent freezing andcontaminant buildup on the emitters, especially at low temperature. Theemitters and gas flow are directed downstream towards objects to beneutralized and parallel to conveying gases present in the test area.

The volumetric flow of gas needed to stabilize a corona discharge willdepend on the purity of the environment before and after injection ofgas. The corona will be stabilized when the concentration of electronattaching gases is about 0.5% in front of the emitter. In purer gases,any injected gas will add to the electron attaching component towardsthe 0.5% goal. In small chambers with circulating flow the ambient levelof electron attaching components may be increased sufficiently tostabilize corona with much lower injection rates than in the single passcase.

Corona induced gas flows within 1 mm of the emitter are near 20 m/s. Gasinjections into this induced gas, as it is carried into a free stream,will produce the negative ions necessary to stabilize the corona. Aninjection rate of about 20 cm 3/min for each needle-type emitter willprovide the necessary carriers for negative-ion formation at higher gasflows. Typical ionizing air blowers, where the exit velocity is about 2m/s, or chambers with fan-driven flows will need only about 0.005%additions of electron attaching gases to the total flow, when the gasesare injected through and around the emitters.

The air injection rate in FIG. 8 for a single emitter is near 1% andshows full stabilization in a single-pass chamber. The superficialvelocity in the chamber is about 1% the superficial velocity used inblowers. Since air contains 20% oxygen, the electron attaching componentis 0.2% or 0.002% when referred to typical gas velocities from blowers.

It will be readily seen by one of ordinary skill in the art that thepresent invention fulfills all of the objects set forth above. Afterreading the foregoing specification, one of ordinary skill will be ableto effect various changes, substitutions of equivalents and variousother aspects of the invention as broadly disclosed herein. It istherefore intended that the protection granted hereon be limited only bythe definition contained in the appended claims and equivalents thereof.

What is claimed is:
 1. A method of achieving static neutralization in agaseous environment that does not have electron attaching componentswhere the mobility of corona generated positive and negative carrierspecies change over time, comprising the step of: injecting apredetermined quantity of electron attaching gas in close proximity to acorona electrode disposed within the gaseous environment.
 2. The methodof claim 1, wherein said injecting step enables negative ions to formaround and stabilize the corona.
 3. The method of claim 2, wherein saidinjected gas is caused to flow around an emitter region and between theconductive or a semiconductive corona electrode and a conductive orsemiconductive counter electrode.
 4. The method of claim 3, wherein theinjection gas entirely surrounds the corona electrode.
 5. The method ofclaim 3, wherein both the corona electrode and the counter electrode arecorona emitters.
 6. The method of claim 1, wherein said gas is clear,dry air and said method is used within one of a semiconductor componenttester and handler.
 7. The method of claim 1, wherein an environment inwhich the ionizer is placed is substantially nitrogen or a noble gas. 8.The method of claim 7, wherein the electron-attaching gas is placed lessthan 5 mm from the corona electrode.
 9. A static neutralizer for use inan ionizer, comprising: (a) a pair of electrodes between which apotential voltage difference is applied, at least one said electrodebeing a conductive or semiconductive corona electrode, said pair ofelectrodes being placed in a gaseous environment that does not haveelectron attaching components; and (b) means for injecting apredetermined quantity of an electron attaching gas in proximity of acorona formed at an emitter region of the electrodes to thereby formnegative ions and stabilize the corona.
 10. The neutralizer of claim 9,wherein a space between the electrodes is filled with an insulatingmaterial.
 11. The neutralizer of claim 10, wherein said pair ofelectrodes surrounded by said insulating material is placed in thegaseous environment.
 12. The neutralizer of claim 11, wherein saidgaseous environment is nitrogen.
 13. The neutralizer of claim 9, whereinsaid electrodes form a hollow emitter assembly between which electrodesthe gas is injected.
 14. The neutralizer of claim 9, wherein saidelectrodes form a needle cavity assembly wherein a gas injection channelsurrounds the corona electrode.
 15. The neutralizer of claim 9, whereinsaid electrodes are formed as a corona electrode set having a pluralityof needle type emitters mounted within a grounded electrode casing, saidneedle electrodes each being capacitively coupled through a metal ringto a high voltage wire in an installation system with the groundedelectrode casing.
 16. A static neutralizer for use with an ionizerhoused in an environmental chamber, comprising: means for injecting anoble gas into said environmental chamber; a pair of electrodes betweenwhich a potential voltage difference is applied, at least one of saidelectrodes being a conductive or semi-conductive corona electrode; andmeans for injecting a predetermined quantity of an electron attachinggas in proximity of a corona formed at an emitter region of theelectrodes to thereby form negative ions and stabilize the corona. 17.The neutralizer of claim 16, wherein said noble gas is nitrogen.
 18. Theneutralizer of claim 16, wherein a space between the electrodes isfilled with an insulating material.
 19. The neutralizer of claim 16,wherein said electrodes form a hollow emitter assembly between which theelectron attaching gas is injected.
 20. The neutralizer of claim 16,wherein said electrodes form a needle cavity assembly and wherein a gasinjection channel surrounds the corona electrode for injecting theelectron attaching gas.